a) Field of the Invention
This invention relates to an ignition timing control system for use with an in-cylinder injection internal combustion engine in which a fuel injection mode can be switched, and especially to an ignition timing control system for an in-cylinder injection internal combustion engine, which can reduce a torque shock upon switching the fuel injection mode.
b) Description of the Related Art
As engines for automotive vehicles, lean-burn engines with a combustion air/fuel ratio set higher than a stoichiometric air/fuel ratio from the viewpoint of an improvement in fuel economy have been proposed in recent years.
A lean-burn engine can generally bring about a greater improvement in fuel economy as its combustion air/fuel ratio becomes higher. Due to deterioration in combustion, however, there is a limitation to the increase in the air/fuel ratio. As a method for raising this limitation as much as possible, a technique has been proposed to perform stratified combustion by making an air-fuel mixture as rich as possible around a point of ignition within a combustion chamber while maintaining a high overall air/fuel ratio within the combustion chamber.
Further, to permit operation of engines with still leaner air-fuel mixtures, in-cylinder injection engines in which fuel is injected directly into cylinders have also been developed. According to these in-cylinder injection engines, fuel can be injected, for example, at a late stage of a compression stroke. Operation of high fuel economy is therefore feasible with an air-fuel mixture extremely lean as a whole while establishing a rich air-fuel mixture only in the vicinity of a spark plug and hence assuring good combustibility by producing a stratified vertical swirl within a cylinder and injecting fuel into the stratified vertical swirl immediately before ignition by the spark plug (for example, at a late stage of a compression stroke).
Upon performing lean-burn operation, the rate of NOx emission is known to decrease as the combustion air/fuel ratio is set higher. In stratified lean-burn operation, however, the air/fuel ratio is relatively low in the vicinity of the point of ignition, thereby imposing a certain limitation on the reduction of the rate of NOx emission.
As a method for achieving such a reduction in the rate of NOx emission, it is contemplated to conduct recirculation of exhaust gas (EGR). For example, a great deal of exhaust gas may be recirculated in a compression-stroke lean fuel injection mode to achieve stabilization-of combustion and a further reduction in the rate of NOx emission.
Further, in such internal combustion engines, target exhaust gas recirculation rates are set according to individual operation modes (fuel injection modes). Upon switching the operation mode, control of an EGR valve is performed so that the exhaust gas recirculation rate can promptly be rendered equal to the corresponding target exhaust gas recirculation rate.
Upon switching the fuel injection mode (operation mode) as described above, an engine torque may vary, leading to production of a shock (torque shock) in some instances.
With a view to overcoming this problem, a target air/fuel ratio A/F is changed by controlling a quantity of air through an air bypass valve (ABV) or an injection quantity of fuel through a fuel injection valve (injector) so that variations in engine torque can be reduced. There is however a delay in the feeding of air into a combustion chamber especially when the fuel injection mode is switched from a compression-stroke fuel injection operation mode to an intake-stroke fuel injection operation mode. Upon such switching of an operation mode, a shock may still not be reduced sufficiently in some instances.